Ensuring this requires that teachers have a great deal of knowledge, skill, and training. Children entering preschool vary significantly in what they know and can do. Some have had rich learning expe¬riences at home, in a program, or both prior to entering preschool; some have not had the kinds of stimulating or supportive environments that contribute to optimal development and learning. Because there is such a range in the types and quality of learning experiences—not to mention individual differences in children's temperament and interests—children enter pre¬school with different developmental strengths.